1. Field of the Invention
The present invention relates to a bridged LAN and, more particularly, to a bridged LAN (Local Area Networks) in which a spanning tree is applied to VLANs (Virtual LANs) and to a communication node apparatus, a terminal apparatus, and a gateway apparatus each applied to the bridged LAN.
2. Description of the Related Art
In recent years, there has been an increase in the number of network configurations each including a plurality of geometrically distributed intra-company LANs connected via a transit network, e.g., a public network. FIG. 18 shows an example of such a network configuration, in which LANs #A1 and #A2 in a company A are connected via a transit network NW. Likewise, LANs #B1 and #B2 in a company B are also connected via the transit network NW. For example, a terminal apparatus, such as a personal computer PC-A1, belonging to the LAN #A1 of the company A performs data communication with a terminal apparatus, such as a personal computer PC-A2, belonging to the LAN #A2 of the company A via the transit network NW.
For improved fault tolerance, the transit network NW typically has a network structure in which a plurality of redundant paths are formed by using a plurality of bridges BRs (BR1 to BR4 in FIG. 18). A network in a configuration in which a plurality of LANs are thus connected via bridges is termed “bridged LAN”. In the transit network NW, when the plurality of bridges are connected in a loop-like shape, there is a case where a frame having a broadcast address as its destination address, such as, e.g., an ARP (Address Resolution Protocol) frame, is continuously forwarded from one bridge to another and a so-called broadcast storm occurs. As a technology for circumventing the broadcast storm, there is an STP (Spanning Tree Protocol) defined in the IEEE 802.1d.
In the STP, the broadcast storm is circumvented by managing such a loop-shaped network as a logically tree-structured network named “spanning tree”. Specifically, in the STP, one of a plurality of bridges forming a loop is selected as a Root bridge in accordance with the value of a bridge ID. Each of the other bridges determines a route to the Root bridge so as to minimize a cost value calculated based on a line speed and clogs the other routes whose cost value are not minimum (“blocking”), whereby the network physically connected in the loop-like shape is managed as a logical spanning tree network.
When there exist a plurality of routes each passing through one of adjacent bridges and having the minimum cost in the direction of the Root bridge, a higher priority is given to one of the routes which passes through the adjacent bridge having a smaller bridge ID. In the STP, data termed “BPDU” for carrying the bridge ID and the cost value determined by the line speed is exchanged between the bridges in the route determining process described above.
FIG. 19 shows an example of a bridged LAN network structure to which the STP is applied. In FIG. 19, four LANs #1 to #4 are connected to each other via bridges BR1 to BR4 connected in a loop-like shape. Although each of the LANs can accommodate a plurality of personal computers, only one personal computer PC is connected to each of the LANs for simple illustration.
Of the bridges BR1 to BR4, the bridge BR1 having a minimum ID serves as a Root bridge. Of three Ethernet ports P11, P12, and P13 in the Root bridge BR1, the ports P11 and P12 are used as designated ports DP. In the bridge BR2, an Ethernet port P21 near the Root bridge BR1 is used as a root port RP and an Ethernet port P22 far from the Root bridge is used as the designated port DP.
In the bridge BR4, an Ethernet port P41 near the Root bridge is used as the root port RP and an Ethernet port P42 far from the Root bridge is used as the designated port DP. In the bridge BR3, an Ethernet port P32 connected to the bridge BR2 is used as the root port RP and an Ethernet port P31 connected to the bridge BR4 is used as a non-designated port NDP, whereby the line between the bridges BR3 and BR4 is blocked. Thus, a spanning tree having the Root bridge as a peak can be constructed in the STP by determining the Root bridge, the root ports, the designated ports, and the non-designated ports.
In recent years, as the scale of a network increases, VLANs (Virtual Local Area Networks) formed by dividing a single LAN into a plurality of virtual LANs are also operated. The VLAN is defined in the IEEE 802.1q. When a network is divided into a plurality of VLANs, it becomes possible to reduce the arrival range (broadcast domain) of a broadcast frame, such as an ARP, and circumvent the congestion of the network bandwidth.
As a technology which implements the STP on the VLAN described above, there is an MST (Multiple Spanning Tree Protocol) defined in the IEEE 802.1. In the MST, a spanning tree is constructed for each of MST instances composed of a single or a plurality of VLANs.
FIG. 20 shows an example of a network structure of a bridged LAN to which the MST is applied.
In the example, LANs #1 to #4 are connected to each other via a transit network NW including bridges BR1 to BR 4 connected in a loop-like shape. Personal computers PC1-1 to PC1-3 are connected to the LAN #1. To the LANs #2 to #4, personal computers PC2, PC3, and PC4 are connected, respectively.
In FIG. 20, two VLANs are formed. The first VLAN #1 is indicated by the solid line and the second VLAN #2 is indicated by the broken line. To the first VLAN #1, the personal computer PC1-1 connected to the LAN #1 and the entire LAN #3 belong. To the second VLAN #2, the personal computers PC1-2 and PC1-3 each connected to the LAN #1 and the entire LANs #2 and #4 belong.
It is assumed here that the VLANs #1 and #2 belong to the MST instances different from each other, which are defined as “MST instance #1” and “MST instance #2”. In the MST, the Root bridge RB is selected for each of the MST instances and the root port RP, the designated DP, and the non-designated port NDP are determined for each of the bridges. In the description given below, it is assumed that the bridges BR1 and BR3 are selected as the respective Root bridges RB in the MST instance #1 and the MST instance #2.
In the bridge BR1, the Ethernet port P11 is the designated port DP in each of the MST instances #1 and #2 and the Ethernet port P12 is the designated port DP in the MST instance #1, while it is the non-designated port NDP in the instance #2. In the bridge BR2, the Ethernet port P21 is the root port RP in the MST instance #1, while it is the designated port DP in the MST instance #2, and the Ethernet port P22 is the designated port DP in each of the MST instances #1 and #2.
In the bridge BR3, the Ethernet port P31 is the non-designated port NDP in the MST instance #1, while it is the designated port DP in the MST instance #2, and the Ethernet port P32 is the root port RP in each of the MST instances #1 and #2. In the bridge BR4, the Ethernet port P41 is the root port RP in each of the MST instances #1 and #2 and the Ethernet port P42 is the designated port DP in the MST instance #1, while it is the root port RP in the MST instance #2. As a result, the line between the bridges BR1 and BR2 is blocked in the MST instance #2 and the line between the bridges BR3 and BR4 is blocked in the MST instance #1.
Thus, the MST can construct respective spanning trees individually in the plurality of VLANs (VLAN #1 and VLAN #2 in FIG. 20) and allows a reduction in the scale of the broadcast domain, which is an advantage of the VLAN, simultaneously with the circumvention of the broadcast storm, which is an advantage of the STP.
In the bridged LAN shown in FIG. 20, it is assumed that the personal computer PC1-2 functions as a WEB server, the personal computer PC1-3 functions as a SIP (Session Initiation Protocol) server for session management, the personal computer PC2 is a client of the WEB and the SIP, and the personal computer PC4 is a client of the SIP. These personal computers PC belong to the VLAN #2. The WEB client can receive a WEB service in accordance with a http protocol from the WEB server (PC1-2). On the other hand, SIP clients can perform VoIP (Voice over IP) communication via the SIP server (PC1-3).
Since the line between the port P12 of the bridge BR1 and the port P21 of the bridge BR2 is blocked in the VLAN #2, when the personal computer PC2 receives a WEB service from the WEB server (PC1-2), the personal computer PC2 communicates with the WEB server via the bridges BR2, BR3, BR4, and BR1. When the personal computer PC2 performs VoIP communication with the personal computer PC4 via the SIP server (PC1-3), the personal computer PC2 is also connected to the SIP server (PC1-3) via a communication route sequentially passing through the bridges BR2, BR3, BR4, and BR1 in this order.
In a communication service such as a WEB service which does not require strict real-time transmission, the number of bridges through which communication frames are forwarded does not cause a particular problem. However, in a voice communication such as VoIP, since a data forwarding delay on a network has to be reduced, it is desired to perform communication along a minimum delay route having a smaller number of bridges through which communication frames have to pass.
In the MST, however, the STP is applied to each of the VLANs individually as described above and it is impossible to construct a different spanning tree for each of services in the transit network. Accordingly, in a case where the personal computer PC2 belonging to the VLAN #2 performs VoIP communication with the personal computer PC4 in FIG. 20, e.g., the personal computer PC2 cannot access the SIP server (PC1-3) via a shortest route passing only through the bridges BR2 and BR1. Thus, according to the existing MST, it is difficult to construct spanning trees different depending on a service type, such as WEB service or VoIP.